Nematodes (sometimes known as “roundworms”) are the most common multi-cellular animals on earth. There are thousands of species. While most species are not harmful to plants, unfortunately, some are plant parasites. Some, such as pin nematodes (Paratylenchus) and stubby-root nematodes (Trichodorus and Paratrichodorus), live outside the plant (that is, they are ectoparasites) pierce root cells with a body part known as the stylet and suck out the content of root cells. Others, such as root-lesion nematodes (Pratylenchus) enter the plant root and move through it, weakening the root system. Root-knot nematodes (Meloidogyne) enter the roots as juveniles and release chemicals which cause the formation of visible galls or “root knots”.
As a group, nematodes cause millions of dollars of damage to turf grasses, corn and other vegetable crops, cotton plants, and fruit trees. Methods of control include rotating crops and application of chemicals. Biological means such as nematophagous fungi (see, e.g., U.S. Pat. No. 6,749,848) have been tried, as have methods such as introducing nucleotide sequences that confer resistance (see, e.g., U.S. Pat. No. 6,284,948). Chemicals such as organophosphates have been the major commercially used technique for controlling nematodes. Unfortunately, they are toxic to wildlife, and their use is being restricted or terminated due to environmental concerns. Nemacur®, the only nematicide approved in various states for use on plants after they have been planted (“post plant”), is highly toxic to fish and wildlife. By agreement with the U.S. Environmental Protection Agency, the manufacturer will not be able to sell it after May 2007.
According to Duke, S.O. “Natural pesticides from plants”, pp. 511-517, in J. Janick and J. E. Simon (eds.), ADVANCES IN NEW CROPS, (Timber Press, Portland, Oreg., 1990), many plant species, such as marigolds, chrysanthemums, castor bean, margosa, and some members of the family Asteraceae are resistant to nematodes. The reference indicates that the active principle for the nematicidal activity has not been discovered in all of these examples and that no plant-derived products were being sold commercially for control of nematodes.
Juglone is a naphtaquinone produced by members of the Juglandaceae, and particularly plants of the genus Juglans. It is exuded particularly by the black walnut; juglone is toxic to a number of commercially valuable plant types, which therefore cannot be grown in proximity to walnut trees. Juglone is only slightly soluble in water and does not travel very far through the soil. Its toxicity to other plants occurs when the roots of the juglone-producing tree come in close proximity to the roots of a susceptible plant.
In 2001, Wuyts et al. presented a poster stating that juglone had some toxicity to the endoparasitic nematode Pratylenchus penetrans. Unfortunately, the inability of juglone to travel far through the soil and its toxicity to many commercially valuable plants makes its use as a nematicide in the field problematic. In 2002, a California state science fair project summary reported that a walnut extract made from steeping cut up branches of walnut in water had some toxicity to root lesion nematodes placed in the extract. In 2003, however, a further California state science fair project summary reported on tests of the ability of methyl bromide, methyl iodide, walnut “tea” extract, and commercial compost to disinfest plant roots of root-knot nematodes. This project summary reported that the walnut tea extract failed to disinfest the soil of the nematodes.
It would be desirable to have alternative means to inhibit nematode infestations of plants.